This invention relates generally to positioning systems using signals broadcast from a plurality of orbiting satellites, and, more particularly, to satellite-based differential positioning systems that determine the position coordinates of a remote receiver relative to a reference receiver having known coordinates.
Satellite-based positioning systems such as the Global Positioning System (GPS) are now a highly popular means of accurately and precisely determining a receiver's coordinates. These systems have numerous practical applications and, depending on the time duration over which measurements are taken, they can determine a receiver's position to sub-centimeter accuracy.
In GPS, a number of satellites orbiting the earth in well-defined polar orbits continuously broadcast signals indicating their precise orbital positions. Each satellite broadcasts two modulated carrier signals, designated L.sub.1 and L.sub.2. The signals from the various satellites are all broadcast on the same two frequencies, but are each modulated by a unique, pseudorandom digital code. Each satellite signal is based on a precision internal clock. The receivers detect the superimposed modulated L.sub.1 and L.sub.2 carrier signals and measure either or both of the code and carrier phase of each detected signal, relative to their own internal clocks. The detected code and carrier phases can be used to determine the receiver's position coordinates.
In absolute positioning systems, i.e., systems that determine a receiver's position coordinates without reference to a nearby reference receiver, the position determination is subject to errors caused by the ionosphere. The ionosphere imposes a group delay on the modulated signals, delaying detection of the modulated code. This makes the broadcasting satellite appear to be further away that it is, in fact. This error can be as much as several hundred meters, although it is more typically on the order of ten meters.
By contrast, the same ionosphere causes a phase advance of the carrier signal, which is equal in magnitude to the delay in the detected code phase. The ionosphere-caused range measurement errors can be corrected by adjusting the L.sub.1 and L.sub.2 code measurements in accordance with a suitable combination of the L.sub.1 and L.sub.2 carrier phase measurements. Such a technique is described in a paper by Ronald R. Hatch, entitled "The Synergism of GPS Code and Carrier Measurements," Magnavox Technical Paper MX-TM-3353-82, Jan. 1982.
Although the ionospheric correction technique referred to above is generally satisfactory in eliminating the ranging errors caused by the ionosphere in a absolute positioning system, it has not proven to be entirely satisfactory. This is because the noise level is increased substantially by the correction procedure and because the procedure generally requires a substantial number of independent measurements to be processed before a sufficiently accurate measurement can be obtained.
Frequently, a reference receiver located at a reference site having known coordinates is available for receiving the satellite signals simultaneously with the receipt of signals by the remote receiver. If the reference and remote receivers are sufficiently close to each other, e.g., within about 50 to 100 kilometers, it can be assumed that the ionosphere affects the various satellite signals they receive substantially equally. In this circumstance, the signals received simultaneously by the two receivers can be suitably combined to substantially eliminate the error-producing effects of the ionosphere and thus provide an accurate determination of the remote receiver's coordinates relative to the reference receiver's coordinates.
To properly combine the signals received simultaneously by the reference receiver and the remote receiver, and thereby eliminate the error-producing effects of the ionosphere, it is necessary to provide an initial estimate of the remote receiver's coordinates. By far the easiest way to obtain the initial relative position of the remote receiver is to locate it at a pre-surveyed marker. Unfortunately, such markers are not always available. An alternative procedure for determining the initial coordinates of the remote receiver relative to those of the reference receiver is to exchange the antennas for the two receivers while both continue to detect the L.sub.1 carrier signals. This results in an apparent movement between the two antenna of twice the vector distance between them. This apparent movement can be halved and used as the initial offset between the two receivers.
Both of the initial relative positioning techniques described above suffer the disadvantage of having to be repeated if the number of L.sub.1 carrier signals being detected ever drops below four, whether due to loss of lock or due to signal path obstruction. This generally requires a substantial amount of time and is, therefore, not considered desirable.
Another approach proposed in the past for determining the initial relative positions of a remote receiver and a reference receiver in a differential positioning system is to constrain the remote receiver to a fixed position until its coordinates can be reestablished to within about ten centimeters of accuracy. This allows the use of routine fixed site
positioning techniques that process the L.sub.1 carrier phase and code measurements. Unfortunately, these techniques generally require at least ten minutes to yield the required accuracy.
It should, therefore, be appreciated that there is a need for an apparatus and technique for determining the initial coordinates of a remote receiver relative to a fixed reference receiver, without imposing any requirements on specific movement of the remote receiver and without requiring an undue amount of time. The present fulfills this need.